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Date: Tue, 29 Oct 2013 18:43:21 -0700
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Subject: Re: MQ Patch.
From: Adrian Chadd <adrian@freebsd.org>
To: Andre Oppermann <andre@freebsd.org>
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Cc: "freebsd-net@freebsd.org" <net@freebsd.org>,
 Luigi Rizzo <rizzo@iet.unipi.it>, Navdeep Parhar <np@freebsd.org>,
 Randall Stewart <rrs@lakerest.net>
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Hi,

We can't assume the hardware has deep queues _and_ we can't just hand
packets to the DMA engine.

Why?

Because once you've pushed it into the transmit ring, you can't
guarantee / impose any ordering on things. You can't guarantee that
you can abort a frame that has been queued because it now breaks the
queue rules.

That's why we don't want to just have a light wrapper around hardware
transmit queues. We give up way too much useful control.

I've seen this both when doing wifi (where I absolutely have to have
per-node, per-TID queues, far before it hits the hardware) and doing
WAN style optimisation, where I want to ensure I only queue a handful
of milliseconds of frames to the hardware so I can ensure I can hit
QoS requirements (eg there being a large amount of bulk data, then I
want to inject some voice traffic that should go out sooner..)

Thanks,


-adrian